1. Field of the Invention
The present invention relates to the fabrication of integrated circuits. More particularly, the present invention relates to improved techniques for increasing circuit density and/or reducing substrate damage in an integrated circuit employing laser fusible links.
2. Description of Related Art
Semiconductor integrated circuits (IC) and their manufacturing techniques are well known. In a typical integrated circuit, a large number of semiconductor devices may be fabricated on a silicon substrate. To achieve the desired functionality, a plurality of conductors are typically provided to couple selected devices together. In some integrated circuits, conductive links are coupled to fuses, which may be cut or blown after fabrication using lasers. In a dynamic random access memory (DRAM) circuit, for example, fuses may be employed to replace failing memory array elements with redundant array elements. In logic circuits, fuses may be used to select or modify circuit performance or functions. Laser fusible links comprise metal lines that can be explosively fused open by application of laser energy which causes a portion of the link material to vaporize and a portion to melt. Typically, the fusible link is thin and composed of aluminum or polysilicon. Or it may be made of the same metals as the chip conductors. In operation a short pulse of laser energy in predetermined arcs (spot) is impinged upon the link.
Since every link is not necessarily blown, it is important to ensure that adjacent fuses are not blown by reflected light. Two methods are currently used to ensure that only the desired fuses are blown and that adjacent fuses are not inadvertently blown. The first method simply spaces the fuses two or three spot diameters apart. The second method builds metal walls between the adjacent fuses. Both those methods result in large fuse pitches and significant use of chip area.
In cases where the fusible links are built from the same material as the chip conductors, become thicker, are made of composite layers including layers of refractory metals (Tungsten and various suicides), or are comprised of highly reflective metals (copper), blowing the fuses with lasers becomes more difficult.
The increasing speed requirements of logic chips are the driving force behind these fusible link materials. More commonly, fuses may be employed to set the enable bit and the address bits of a redundant array element in a DRAM circuit.
FIG. 1 illustrates a typical dynamic random access memory integrated circuit, having a main memory array 102. To facilitate replacement of a defective main array element within the main memory array 102, a redundant array 104 is provided as shown. A plurality of fuses in fuse array 106 are coupled to redundant array 104 via a fuse latch array 108 and a fuse decoder circuit 110. To replace a defective main memory array element, individual fuses in fuse array 106 may be blown or cut to set their values to either a "1" or a "0" as required by the decoder circuit 110.
During operation, the values of the fuses in fuse array 106 are typically loaded into fuse latch array 108 upon power up. These values are then decoded by fuse decoder circuit 110 during run time, thereby facilitating the replacement of specific failed main memory array elements with specific redundant elements of redundant array 104. Techniques for replacing failed main memory array elements with redundant array elements are well known in the art and will not be discussed in great detail here.
As mentioned earlier, the fuse links within fuse array 106 may be selectively blown or cut with a laser beam. Once blown by the laser beam, the fuse changes from a highly conductive state to a highly resistive (i.e., non-conductive) state. A blown fuse inhibits current from flowing through and represents an open circuit to the current path. With reference to FIG. 2A, fuse links 202, 204, 206, and 208 of fuse array element 106 are shown in their unblown (i.e., conductive) state. In FIG. 2B, a laser beam has been employed to cut or blow fuse link 204, thereby inhibiting the flow of current there through.
It has been found that the use of a laser beam to set, cut or blow a fuse link may render the area under the fuse link or adjacent fusible links vulnerable to laser-induced damage, mainly due to the absorption of laser energy during the fuse setting operation. Because of the possibility of laser-induced damage, the areas underlying the fuse links are typically devoid of semiconductor devices (e.g., transistors) and the fuses are spaced far apart in conventional systems.
Even when there are no active devices beneath the fusible link or other closely spaced fusible links, the substrate itself may also experience some degree of laser-induced damage. This is because silicon, which is the typical substrate material, absorbs the laser energy readily, particularly when short wavelength lasers are employed. For this reason, lasers having relatively long wavelengths such as infrared lasers have been employed in conventional systems for the fuse setting operation.
Even though infrared lasers are helpful in minimizing laser-induced damage to the underlying substrate, the use of lasers having relatively long wavelengths involves certain undesirable compromises. By way of example, the relatively long wavelength of the infrared laser forms a relatively large spot on the substrate during the fuse setting operation, which limits how closely the fuse links can be packed next to one another. For infrared lasers having a wavelength of, for example, about 1micron, the spot created on the substrate may be two times the wavelength or about 2 to 2.5 microns wide.
The disadvantages associated with lasers having relatively long wavelengths is illustrated with reference to FIGS. 3A and 3B below. FIG. 3A is a cross-sectional view of a portion of fuse array 106, including fuse links 202, 204, 206, and 208. In FIG. 3A, fuse links 202, 204, 206, and 208 are shown encapsulated within a passivation layer 302. A substrate 304 underlies the fuse links as shown. It should be noted that the illustration of FIG. 3A is highly simplified to facilitate illustration and fuse array 106 may include other conventional layers and/or components as is known.
In FIG. 3B, fuse link 204 of FIG. 3A has been blown or cut using a laser beam. In place of fuse link 204, a void 310 exists, whose diameter C is roughly twice the wavelength of the laser beam employed. The diameter C of the laser spot places a lower limit on the minimum fuse pitch 312 between adjacent fuse links. If the fuses are placed too closely together for a given laser wavelength, an adjacent fuse link may be inadvertently blown or cut, rendering the IC defective.
Using a laser with a shorter wavelength would reduce the diameter C of the laser spot and concomitantly the minimum fuse pitch. However, a shorter wavelength laser substantially increases the likelihood of underlying substrate damage in conventional systems since the silicon substrate absorbs laser energy from shorter wavelength lasers much more readily. If a shorter wavelength laser is employed to set the fuse links of conventional systems fuse array 106, excessive substrate damage in area 320 of substrate 304 may result, possibly leading to integrated circuit defects and failure.
In view of the foregoing, there is a conventional need for improved techniques for fabricating integrated circuits having laser fusible links. More particularly, there is a conventional need for improved laser fuse link structures and methods therefor, which advantageously minimize substrate damage during the fuse setting operation and/or permit the use of shorter wavelength lasers to reduce the fuse pitch.